Genetically modified rodent with an inducible ACVR1 mutation in exon 7 that causes ectopic bone formation

ABSTRACT

A genetically modified rodent is provided that comprises a modified Acvr1 gene that comprises a conditional altered exon 7 encoding R258G in anti sense orientation, flanked by site-specific recombinase recognition sites, wherein the altered exon is inverted to sense orientation upon action of a recombinase, resulting in ectopic bone formation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefits of priority from U.S. ProvisionalApplication No. 62/684,582, filed Jun. 13, 2018, and U.S. ProvisionalApplication No. 62/828,532, filed Apr. 3, 2019, the entire contents ofwhich are incorporated herein by reference.

FIELD OF THE DISCLOSURE

This disclosure relates to genetically modified rodent animals androdent models of human diseases. More specifically, this disclosurerelates to genetically modified rodents whose genome comprises amodified rodent Acvr1 gene; rodents that exhibit a phenotypical featureof fibrodysplasia ossificans progressiva (FOP) such as ectopic boneformation; isolated rodent tissues and cells whose genome comprises amodified rodent Acvr1 gene; isolated nucleic acids that comprise amodified rodent Acvr1 gene; compositions and methods of making thegenetically modified rodents; methods of breeding; and methods of usingthe genetically modified rodents.

INCORPORATION BY REFERENCE OF SEQUENCE LISTING

The sequence listing in the ASCII text file, named as36190_10461US01_SequenceListing.txt of 4 KB, created on Jun. 6, 2019,and submitted to the United States Patent and Trademark Office viaEFS-Web, is incorporated herein by reference.

BACKGROUND ART

Various publications, including patents, patent applications, publishedpatent applications, accession numbers, technical articles and scholarlyarticles are cited throughout the specification. Each of these citedpublications is incorporated by reference, in its entirety and for allpurposes, in this document.

Acrv1 is a type I receptor for bone morphogenic proteins (BMPs). Certainmutations in the human ACVR1 gene, including mutations that give rise tothe amino acid modification R206H or R258G, are strongly associated withthe disease fibrodysplasia ossificans progressiva (FOP) (see, e.g., USPat. Appl. Publ. No. 2009/0253132; Pignolo, R. J. (2011) OrphanetJournal of Rare Diseases, 6:80, 1-6; and Kaplan et al., Am J Med GenetA. 2015; 167(10): 2265-2271). Chimeric mice that bear an R206H mutationin Acvr1 develop an FOP-like phenotype (see, e.g., Chakkalakal et al,(2012) J. Bone and Mineral Res. 27:1746-1756). Certain mutations in theAcvr1 gene, e.g., those resulting in an R206H Acvr1 protein variant, areperinatal lethal in mice and present challenges for passing a knock-ingene comprising the mutation through the germline of a rodent.

SUMMARY OF THE DISCLOSURE

This disclosure is directed to genetically modified rodent animals thatcomprise in their germline a nucleic acid sequence that comprises amodified rodent Acvr1 gene.

Genetically modified rodent animals are provided that comprise in theirgermline a nucleic acid sequence that comprises a modified rodent Acvr1gene, wherein the modified rodent Acvr1 gene comprises a conditionalalteration of a rodent Acvr1 gene, wherein the alteration renders therodent animal susceptible to ectopic bone formation.

Genetically modified rodent animals are provided that comprise in theirgermline a nucleic acid sequence that comprises a modified rodent Acvr1gene comprising a conditional altered Acvr1 exon, wherein induction ofexpression of the conditional altered Acvr1 exon confers upon the rodentanimal a susceptibility to ectopic bone formation. In one embodiment,the altered Acvr1 exon is exon 7. In a specific embodiment, the alteredAcvr1 exon 7 includes an alteration resulting in a R258G amino acidvariation in Acvr1 protein.

Rodent animals are provided that conditionally express an altered Acvr1allele. In various aspects, the altered Acvr1 allele is an allele thatconfers a pathological phenotype on the rodent animal expressing theallele. In various aspects, the rodent animals comprise an altered exonof an Acvr1 allele flanked upstream and downstream with site-specificrecombinase recognition sites (SRRS′s), and the rodent animal comprisesa recombinase that recognizes the SRRS′s, wherein the activity of therecombinase is inducible.

Rodent animals are provided that comprise a modification of a rodent Acml gene that causes (in one embodiment, in a heterozygote; in oneembodiment, in a homozygote), promotes, or makes the rodent animalsusceptible to ectopic bone formation

Rodent animals are provided that comprise a conditional alteration of arodent Acvr1 gene, wherein an altered allele is not expressed in utero,and is not expressed perinatally, and wherein the rodent animals expressthe altered Acvr1 allele in a conditional manner, wherein theconditional expression is induced by administration of a compound ofinterest to the rodent animal.

In one aspect, a rodent animal is provided whose genome comprises amodified rodent Acvr1 locus comprising an altered Acvr1 exon inantisense orientation, wherein the altered Acvr1 exon is flankedupstream and downstream by SRRS′s that are oriented to direct aninversion when acted upon by a recombinase that recognizes the SRRS′s.

In some embodiments, a rodent animal is provided whose genome comprisesa modified rodent Acvr1 gene within an endogenous rodent Acyl locus, themodified rodent Acvr1 gene comprising a functioning Acvr1 exon 7 insense orientation and flanked upstream and downstream by a first pair ofSRRS′, and an altered Acvr1 exon 7 in antisense orientation and flankedupstream and downstream by a second pair of SRRS′, wherein the first andsecond pairs of SRRS′ are orientated such that a recombinase can invertthe altered Acvr1 exon 7 into sense orientation and delete thefunctioning Acvr1 exon 7, resulting in an altered Acvr1 allele (i.e., anAcvr1 allele comprising the alteration). In various embodiments, exceptfor exon 7, the remaining exons of a modified rodent Acyl gene arefunctioning exons of an endogenous rodent Acvr1 gene, e.g., wild typerodent exons present at an endogenous rodent Acvr1 locus. In variousembodiments, the altered Acvr1 allele is an allele that confers apathological phenotype on the rodent animal expressing the allele.

In some embodiments, provided herein is a genetically modified rodentwhose genome comprises a modified rodent Acvr1 gene within an endogenousrodent Acvr1 locus, wherein the modified rodent Acvr1 gene comprises (a)a functioning Acvr1 exon 7 encoding R258 in sense orientation, flankedupstream and downstream by a first pair of SRRS′; and (b) an alteredrodent Acvr1 exon 7 encoding a R258G variation in anti senseorientation, flanked by a second pair of SRRS′ that are different fromthe first pair of SRRS′; wherein the first and second pairs of SRRS′ areoriented so that a recombinase can invert the altered rodent Acvr1 exon7 into sense orientation, delete the functioning Acvr1 exon 7, and allowan altered Acvr1 allele comprising the altered rodent Acvr1 exon 7 to beexpressed. In some embodiments, expression of an altered Acvr1 allelecomprising the altered rodent Acvr1 exon 7 results in ectopic boneformation. In various embodiments, except for exon 7, the remainingexons of a modified rodent Acvr1 gene are functioning exons of anendogenous rodent Acvr1 gene, e.g., wild type rodent exons present at anendogenous rodent Acvr1 locus.

In some embodiments, a functioning Acvr1 exon 7 encoding R258 is anaturally occurring, functioning rodent Acvr1 exon 7, i.e., a wild typerodent Acvr1 exon 7. In some embodiments, a functioning Acvr1 exon 7encoding R258 is a substantially human ACM1 exon 7. In some embodiments,a substantially human ACVR1 exon 7 is a naturally occurring, functioninghuman ACVR1 exon 7, i.e., a wild type human ACVR1 exon 7. In otherembodiments, a substantially human ACVR1 exon 7 differs from a wild typehuman ACVR1 exon 7 by at least one nucleotide (i.e., one or morenucleotides) and has a reduced sequence identity with an altered rodentAcvr1 exon 7 as compared to the sequence identity between the wild typehuman ACVR1 exon 7 and the altered rodent Acvr1 exon 7. In someembodiments, a substantially human ACVR1 exon 7 encodes the same aminoacids as a wild type human ACVR1 exon 7.

In some embodiments, the first pair of SRRS′ includes a first SRRS and asecond SRRS, wherein the first and second SRRS′ are compatible with eachother and are oriented to direct an inversion. In some embodiments, thesecond pair of SRRS′ includes a third SRRS and a fourth SRRS, whereinthe third and fourth SRRS′ are compatible with each other, are orientedto direct an inversion, but are not compatible with the first or secondSRRS.

In some embodiments, a genetically modified rodent is provided thatcomprises a modified rodent Acvr1 gene at an Acvr1 locus in the germlineof the rodent, wherein the modified rodent Acvr1 gene comprises, withrespect to the direction of transcription of the Acvr1 gene, (i) afunctioning exon 7 (e.g., a substantially human exon 7) in senseorientation, and (ii) an altered exon 7 in anti sense orientation; andcomprises a first pair of SRRS′ composed of a first SRRS (SRRS1) and asecond SRRS (SRRS2), and a second pair of SRRS′ composed of a third SRRS(SRRS3) and a fourth SRRS (SRRS4); wherein SRRS1 is compatible withSRRS2, SRRS3 is compatible with SRRS4, but neither SRRS nor SRRS2 iscompatible with SRRS3 or SRRS4, wherein SRRS1 is located upstream of thefunctioning exon 7, SRRS2 is located just downstream (with respect totranscriptional direction of the Acvr1 gene) of the antisense alteredexon 7, wherein SRRS1 and SRRS2 are oriented to direct an inversion;wherein SRRS3 is disposed between the functioning exon 7 and the alteredantisense exon 7, and SRRS4 is located downstream (with respect to thedirection of orientation of the Acvr1 gene) of SRRS2, wherein SRRS3 andSRRS4 are oriented to direct an inversion. In some embodiments, each ofSRRS′ 1-4 is recognized by the same recombinase, such as Cre.

In some embodiments, the first pair of SRRS′ is a pair of Lox2372 sites,and the second pair of SRRS′ is a pair of LoxP sites. In otherembodiments, the first pair of SRRS′ is a pair of LoxP sites, and thesecond pair of SRRS′ is a pair of Lox2372 sites.

In some embodiments, a genetically modified rodent animal furthercomprises an inducible recombinase which recognizes the SRRS′s and iscapable of inverting the antisense altered Acvr1exon to senseorientation. In one embodiment, the gene encoding the induciblerecombinase is in the germline of the rodent, e.g., integrated at anendogenous rodent ROSA26 locus.

Inducibility of a recombinase can be achieved at the transcription (geneexpression) level, or at the protein activity level. In someembodiments, expression of the recombinase gene is inducible, and/ordevelopmentally regulated, and/or specific to certain tissue or celltypes. In other embodiments, expression of the recombinase gene isconstitutive, and the activity of the recombinase is inducible.

In some embodiments, the recombinase is Cre. In some embodiments, theSRRS′s are lox sites or variants thereof which are recognized by Cre. Insome embodiments, a recombinase (e.g., Cre) is fused to a ligand bindingdomain responsive to binding by a ligand. In some embodiments, arecombinase (e.g., Cre) is fused to a ligand binding domain that is theligand binding domain of a receptor such as a steroid receptor, aglucocorticoid receptor, a retinoid receptor, a thyroid receptor, or anestrogen receptor (ER), or that is derived from the ligand bindingdomain of a receptor. In some embodiments, Cre is fused to an ERligand-binding domain comprising 12 mutations (referred to herein asCre-ER^(T2)). A Cre-ER^(T2) fusion protein is responsive to (i.e.,activated by) binding by a ligand, e.g., tamoxifen or a functionalanalogs/derivative thereof. In one embodiment, a gene encodingCreER^(T2) is present at the ROSA26 locus of a genetically modifiedrodent.

In some embodiments, a mutant rodent is provided that is derived from agenetically modified rodent described above (i.e., derived from agenetically modified rodent comprising a modified rodent Acvr1 genewhich comprises a conditional Acvr1 alteration in antisenseorientation), wherein the mutant rodent has a genome that comprises theAcvr1 alteration in sense orientation and wherein an Acvr1 genecomprising the alteration is expressed in the mutant rodent which, insome embodiments, results in ectopic bone formation.

In one embodiment, the genetically modified rodent is a mouse having thegenotype Acva1^([R258B]F1Ex/+); GT(ROSA26)Sor^(CreERt2/+).

In some embodiments, a genetically modified rodent is provided thatexpresses an Acvr1 allele comprising a functioning Acvr1 exon 7 (e.g.,in utero and perinatally), wherein upon action of the geneticallymodified rodent by a recombinase, the rodent expresses a variant Acvr1protein comprising a R258G amino acid variation.

In some embodiments, an adult rodent is provided that expresses an Acvr1gene product characterized by a R258G variation, wherein at least 40%,50%, 60%, 70%, 80%, 90%, 95%, or 99% of the cells of the rodent comprisean altered Acvr1 gene encoding the R258G variation. In otherembodiments, an adult rodent is provided that expresses an Acvr1 geneproduct characterized by a R258G variation in specific tissues or celltypes.

In some embodiments, a genetically modified rodent is provided, whereinthe rodent comprises a modified rodent Acvr1 locus in its germline that,upon action by a recombinase, expresses an Acvr1 protein that comprisesa R258G variation.

In some embodiments, a rodent is provided that expresses a proteinvariant comprising a R258G variation, wherein the rodent is non-chimeric(i.e., all the cells in the rodent express the variant protein). In someembodiments, a rodent is provided that expresses a protein variantcomprising a R258G variation, wherein at least 40%, 50%, 60%, 70%, 80%,90%, 95%, or 99% of the cells of the rodent express the protein variant.

In some embodiments, a rodent is provided that expresses a proteinvariant from a modified rodent Acvr1 locus in the germline of therodent, wherein all Acvr1-expressing cells of the rodent comprise amodified rodent Acvr1 gene that encodes an Acvr1 protein that comprisesan R258G variation. In one embodiment, all germ cells of the rodentcomprise a modified rodent Acrv1 locus comprising a conditional geneticmodification that encodes an Acvr1 protein with an R258G variation.

In some embodiments, a genetically modified rodent is heterozygous forthe genetic modification, i.e., the modified rodent Acvr1 genecomprising a conditional alteration. In some embodiments, the rodent ishomozygous for the genetic modification.

In various embodiments, a genetically modified rodent is selected fromthe group consisting of a mouse, a rat, and a hamster. In some specificembodiments, the rodent is a mouse. In some specific embodiments, therodent is a rat.

In another aspect, disclosed herein is a nucleic acid comprising amodified rodent Acvr1 gene described herein.

In some embodiments, a modified rodent Acvr1 gene in a nucleic acidcomprises a functioning Acvr1 exon 7 in sense orientation, flankedupstream and downstream by a first pair of SRRS, and an altered Acvr1exon 7 in antisense orientation, flanked upstream and downstream by asecond pair of SRRS, wherein the first and second pairs of SRRS′ areorientated such that a recombinase can invert the altered Acvr1 exon 7into sense orientation, and delete the functioning Acvr1 exon 7. Invarious embodiments, except for exon 7, the remaining exons of amodified rodent Acvr1 gene are functioning exons of a rodent Acvr1 gene,e.g., wild type exons of an endogenous rodent Acvr1 gene.

In some embodiments, a modified rodent Acvr1 gene in a nucleic acidcomprises (a) a functioning Acvr1 exon 7 encoding R258 in senseorientation, flanked upstream and downstream by a first pair of SRRS′;and (b) an altered rodent Acvr1 exon 7 encoding a R258G variation inantisense orientation, flanked by a second pair of SRRS′ that aredifferent from the first pair of SRRS; wherein the first and secondpairs of SRRS′ are oriented so that a recombinase can invert the alteredrodent Acvr1 exon 7 into sense orientation, delete the functioning Acvr1exon 7, and allow an altered Acvr1 allele comprising the altered rodentAcvr1 exon 7 to be expressed. In some embodiments, expression of analtered Acvr1 allele comprising the altered rodent Acvr1 exon 7 resultsin ectopic bone formation. In various embodiments, except for exon 7,the remaining exons of a modified rodent Acvr1 gene are functioningexons of a rodent Acvr1 gene, e.g., wild type exons of a rodent Acvr1gene.

In some embodiments, a functioning Acyl exon 7 encoding R258 is a wildtype rodent Acvr1 exon 7. In some embodiments, a functioning Acvr1 exon7 encoding 8258 is a substantially human ACVR1 exon 7. In someembodiments, a substantially human ACVR1 exon 7 is a wild type humanACVR1 exon 7. In other embodiments, a substantially human ACVR1 exon 7differs from a wild type human ACVR1 exon 7 by at least one nucleotideand has a reduced sequence identity with an altered rodent Acvr1 exon 7as compared to the sequence identity between the wild type human ACVR1exon 7 and the altered rodent Acvr1 exon 7. In some embodiments, asubstantially human ACVR1 exon 7 encodes the same amino acids as a wildtype human ACVR1 exon 7.

In some embodiments, the first pair of SRRS′ includes a first SRRS and asecond SRRS, wherein the first and second SRRS′ are compatible with eachother and are oriented to direct an inversion. In some embodiments, thesecond pair of SRRS′ includes a third SRRS and a fourth SRRS, whereinthe third and fourth SRRS′ are compatible with each other, are orientedto direct an inversion, but are not compatible with the first or secondSRRS.

In some embodiments, a modified rodent Acvr1 gene in a nucleic acidcomprises, with respect to the direction of transcription of the Acvr1gene, (i) a functioning exon 7 (e.g., a substantially human exon 7) insense orientation, and (ii) an altered exon 7 in antisense orientation;and comprises a first pair of SRRS′ composed of a first SRRS (SRRS1) anda second SRRS (SRRS2), and a second pair of SRRS′ composed of a thirdSRRS (SRRS3) and a fourth SRRS (SRRS4); wherein SRRS1 is compatible withSRRS2, SRRS3 is compatible with SRRS4, but neither SRRS1 nor SRRS2 iscompatible with SRRS3 or SRRS4, wherein SRRS1 is located upstream of thefunctioning exon 7, SRRS2 is located just downstream (with respect totranscriptional direction of the Acvr1 gene) of the antisense alteredexon 7, wherein SRRS1 and SRRS2 are oriented to direct an inversion;wherein SRRS3 is disposed between the functioning exon 7 and the alteredantisense exon 7, and SRRS4 is located downstream (with respect to thedirection of orientation of the Acvr1 gene) of SRRS2, wherein SRRS3 andSRRS4 are oriented to direct an inversion. In some embodiments, each ofSRRS′ 1-4 is recognized by the same recombinase, such as Cre.

In another aspect, disclosed herein is a rodent genome comprising anucleic acid that comprises a modified rodent Acvr1 gene, describedabove.

In some embodiments, the rodent genome further comprises a gene encodingan inducible recombinase that recognizes the SRRS′ and is capable toinverting an altered exon 7 and delete the functioning exon 7.

In some embodiments, the recombinase is Cre. In some embodiments, theSRRS′s are lox sites or variants thereof which are recognized by Cre. Insome embodiments, a recombinase (e.g., Cre) is fused to a ligand bindingdomain responsive to binding by a ligand, e.g., Cre-ER^(T2) which isresponsive to (i.e., activated by) binding by tamoxifen or a functionalanalogs/derivative thereof. In one embodiment, a gene encodingCreER^(T2) is integrated into the ROSA26 locus of a rodent genome.

In a further aspect, disclosed herein is an isolated rodent tissue orcell whose genome comprises a nucleic acid described above, i.e., thegenome of the tissue or cell comprises a modified rodent Acvr1 gene. Insome embodiments, an isolated rodent tissue or cell comprises a modifiedAcvr1 gene within an endogenous rodent Acvr1 locus. In some embodiments,the isolated rodent cell is an embryonic stem (ES) cell. In someembodiments, an isolated rodent tissue or cell is a rodent egg or arodent embryo. The isolated rodent tissue or cell include, for example,mouse or rat tissue or cell.

In some embodiments, an isolated rodent tissue or cell comprises in itsgenome a modified rodent Acvr1 gene comprising a functioning Acvr1 exon7 in sense orientation and flanked upstream and downstream by a firstpair of SRRS′, and an altered Acvr1 exon 7 in antisense orientation andflanked upstream and downstream by a second pair of SRRS, wherein thefirst and second pairs of SRRS′ are orientated such that a recombinasecan invert the altered Acvr2 exon 7 into sense orientation, and deletethe functioning Acvr1 exon 7. In various embodiments, except for exon 7,the remaining exons of a modified rodent Acvr1 gene are functioningexons of an endogenous rodent Acvr1 gene, e.g., wild type exons presentat an endogenous rodent Acvr1 locus.

In some embodiments, an isolated rodent tissue or cell comprises in itsgenome a modified rodent Acvr1 gene comprising (a) a functioning Acvr1exon 7 encoding 8258 in sense orientation, flanked upstream anddownstream by a first pair of SRRS′; and (b) an altered rodent exon 7encoding a R258G variation in anti sense orientation, flanked by asecond pair of SRRS′ that are different from the first pair of SRRS′;wherein the first and second pairs of SRRS′ are oriented so that arecombinase can invert the altered rodent Acvr1 exon 7 into senseorientation, delete the functioning Acvr1 exon 7, and allow an alteredAcvr1 allele comprising the altered rodent Acvr1 exon 7 to be expressed.In some embodiments, expression of an altered Acvr1 allele comprisingthe altered rodent Acvr1 exon 7 results in ectopic bone formation. Invarious embodiments, except for exon 7, the remaining exons of amodified rodent Acvr1 gene are functioning exons of an endogenous rodentAcvr1 gene, e.g., wild type exons present at an endogenous rodent Acvr1locus.

In some embodiments, a functioning Acvr1 exon 7 encoding R258 is a wildtype rodent Acvr1 exon 7. In some embodiments, a functioning Acvr1 exon7 encoding 8258 is a substantially human ACVR1 exon 7. In someembodiments, a substantially human ACM/exon 7 is a wild type human ACVR1exon 7. In other embodiments, a substantially human ACVR1 exon 7 differsfrom a wild type human ACVR1 exon 7 by at least one nucleotide and has areduced sequence identity with an altered rodent Acvr1 exon 7 ascompared to the sequence identity between the wild type human ACVR1 exon7 and the altered rodent Acvr1 exon 7. In some embodiments, asubstantially human ACVR1 exon 7 encodes the same amino acids as a wildtype human ACVR1 exon 7.

In some embodiments, the first pair of SRRS′ includes a first SRRS and asecond SRRS, wherein the first and second SRRS′ are compatible with eachother and are oriented to direct an inversion. In some embodiments, thesecond pair of SRRS′ includes a third SRRS and a fourth SRRS, whereinthe third and fourth SRRS′ are compatible with each other, are orientedto direct an inversion, but are not compatible with the first or secondSRRS.

In some embodiments, an isolated rodent tissue or cell comprises in itsgenome a modified rodent Acvr1 gene comprising, with respect to thedirection of transcription of the Acvr1 gene, (i) a functioning exon 7(e.g., a substantially human exon 7) in sense orientation, and (ii) analtered exon 7 in antisense orientation; and comprises a first pair ofSRRS′ composed of a first SRRS (SRRS1) and a second SRRS (SRRS2), and asecond pair of SRRS′ composed of a third SRRS (SRRS3) and a fourth SRRS(SRRS4); wherein SRRS1 is compatible with SRRS2, SRRS3 is compatiblewith SRRS4, but neither SRRS1 nor SRRS2 is compatible with SRRS3 orSRRS4, wherein SRRS1 is located upstream of the functioning exon 7,SRRS2 is located just downstream (with respect to transcriptionaldirection of the Acvr1 gene) of the antisense altered exon 7, whereinSRRS1 and SRRS2 are oriented to direct an inversion; wherein SRRS3 isdisposed between the functioning exon 7 and the altered anti sense exon7, and SRRS4 is located downstream (with respect to the direction oforientation of the Acvr1 gene) of SRRS2, wherein SRRS3 and SRRS4 areoriented to direct an inversion. In some embodiments, each of SRRS′ 1-4is recognized by the same recombinase, such as Cre.

In a further aspect, disclosed herein is a nucleic acid construct fortargeted modification of an Acvr1 gene in a rodent genome, also referredto as “a targeting nucleic acid”.

In some embodiments, a targeting nucleic acid comprises (a) afunctioning Acvr1 exon 7 encoding R258 in sense orientation, flankedupstream and downstream by a first pair of SRRS′; and (b) an alteredrodent Acvr1 exon 7 encoding a R258G variation in antisense orientation,flanked by a second pair of SRRS′ that are different from the first pairof SRRS′; wherein the first and second pairs of SRRS′ are oriented sothat a recombinase can invert the altered rodent Acvr1 exon 7 into senseorientation and delete the functioning Acvr1 exon 7. In someembodiments, the functioning exon 7 and the altered exon 7 are eachflanked by intronic sequences. Typically a targeting nucleic acidconstruct comprises 5′ and 3′ homology arms (nucleotide sequenceshomologous to the nucleotide sequences at the locus to be targeted) thatmediate homologous recombination and integration of the nucleic acidsequence between the homology arms. In some embodiments, a targetingnucleic acid construct also comprises a selection marker gene tofacilitate identification and selection of correctly targeted clones.

In some embodiments, a functioning Acvr1 exon 7 encoding 8258 is a wildtype rodent Acvr1 exon 7. In some embodiments, a functioning Acvr1 exon7 encoding 8258 is a substantially human ACVR1 exon 7. In someembodiments, a substantially human ACM/exon 7 is a wild type human ACVR1exon 7. In other embodiments, a substantially human ACVR1 exon 7 differsfrom a wild type human ACVR1 exon 7 by at least one nucleotide and has areduced sequence identity with an altered rodent Acvr1 exon 7 ascompared to the sequence identity between the wild type human ACVR1 exon7 and the altered rodent Acvr1 exon 7. In some embodiments, asubstantially human ACVR1 exon 7 encodes the same amino acids as a wildtype human ACVR1 exon 7.

In some embodiments, the first pair of SRRS′ includes a first SRRS and asecond SRRS, wherein the first and second SRRS′ are compatible with eachother and are oriented to direct an inversion. In some embodiments, thesecond pair of SRRS′ includes a third SRRS and a fourth SRRS, whereinthe third and fourth SRRS′ are compatible with each other, are orientedto direct an inversion, but are not compatible with the first or secondSRRS.

In some embodiments, a targeting nucleic acid comprises, with respect tothe direction of transcription of the Acvr1 gene, (i) a functioning exon7 (e.g., a substantially human exon 7) in sense orientation, and (ii) analtered exon 7 in anti sense orientation; and comprises a first pair ofSRRS′ composed of a first SRRS (SRRS1) and a second SRRS (SRRS2), and asecond pair of SRRS′ composed of a third SRRS (SRRS3) and a fourth SRRS(SRRS4); wherein SRRS1 is compatible with SRRS2, SRRS3 is compatiblewith SRRS4, but neither SRRS1 nor SRRS2 is compatible with SRRS3 orSRRS4, wherein SRRS1 is located upstream of the functioning exon 7,SRRS2 is located just downstream (with respect to transcriptionaldirection of the Acvr1 gene) of the antisense altered exon 7, whereinSRRS1 and SRRS2 are oriented to direct an inversion; wherein SRRS3 isdisposed between the functioning exon 7 and the altered antisense exon7, and SRRS4 is located downstream (with respect to the direction oforientation of the Acvr1 gene) of SRRS2, wherein SRRS3 and SRRS4 areoriented to direct an inversion. In some embodiments, each of SRRS′ 1-4is recognized by the same recombinase, such as Cre.

In another aspect, disclosed herein is a method of making a geneticallymodified rodent, comprising modifying a rodent genome to comprise amodified rodent Acvr1 gene within an endogenous rodent Acvr1 locus asdescribed herein.

In some embodiments, the rodent genome is modified by a processcomprising introducing into a rodent embryonic stem (ES) cell atargeting nucleic acid construct described herein; obtaining a rodent EScell whose genome comprises a modified rodent Acvr1 gene; and making agenetically modified rodent by using the rodent ES cell comprising amodified genome. For example, a targeting nucleic acid is introducedinto a rodent ES cell, wherein the targeting nucleic acid comprises afunctioning Acvr1 exon 7 encoding R258 in sense orientation, flankedupstream and downstream by a first pair of SRRS; and (b) an alteredrodent Acvr1 exon 7 encoding a R258G variation in antisense orientation,flanked by a second pair of SRRS′ that are different from the first pairof SRRS′; wherein the first and second pairs of SRRS′ are oriented sothat a recombinase can invert the altered rodent Acvr1 exon 7 into senseorientation and delete the functioning Acvr1 exon 7. A rodent ES cellcan be selected whose genome has been modified and comprises thefunctioning Acvr1 exon 7 encoding 8258 in sense orientation, flankedupstream and downstream by a first pair of SRRS′; and the altered rodentAcvr1 exon 7 encoding a R258G variation in antisense orientation,flanked by a second pair of SRRS′ that are different from the first pairof SRRS′. Such rodent ES cell can be used to make a rodent.

In some embodiments, a genetically modified rodent made by the presentmethod and comprising a modified rodent Acvr1 gene further comprises aninducible recombinase (e.g., Cre). In some embodiments, a nucleic acidencoding an inducible recombinase is present in the genome of a rodentES cell into which a targeting nucleic acid construct is introduced. Insome embodiments, a rodent comprising a modified rodent Acvr1 gene inits genome is made first and then crossed with another rodent comprisinga nucleic acid encoding an inducible recombinase in its genome.

In some embodiments, the inducible recombinase is activated in a rodentand acts to invert an altered exon 7 and delete the functioning exon toallow an Acvr1 protein comprising an alteration (e. R258G) to beexpressed.

In a further aspect, disclosed herein is a method of breeding rodentsand rodent progenies obtained.

In some embodiments, disclosed herein is a method comprising breeding afirst rodent whose genome comprises a modified rodent Acvr1 gene with asecond rodent, resulting in a progeny rodent whose genome comprises themodified rodent Acvr1 gene. A modified rodent Acvr1 gene has beendescribed herein; for example, a modified rodent Acvr1 gene may comprisea functioning Acvr1 exon 7 in sense orientation and flanked upstream anddownstream by a first pair of SRRS′, and an altered rodent Acvr1 exon 7in antisense orientation and flanked upstream and downstream by a secondpair of SRRS′, wherein the first and second pairs of SRRS′ areorientated such that a recombinase can invert the altered Acvr1 exon 7into sense orientation, and delete the functioning Acvr1 exon 7. In someembodiments, an altered rodent Acvr1 exon 7 encodes a R258G variation,and a functioning Acvr1 exon 7 is a substantially human ACVR1 exon 7encoding R258.

In some embodiments, the second rodent comprises an induciblerecombinase. In some embodiments, the inducible recombinase is aninducible Cre recombinase. In some embodiments, an inducible Crerecombinase is a Cre-ER^(T2) recombinase, inducible by binding of aligand such as tamoxifen or a functional derivative or analog thereof.

In some embodiments, the progeny rodent comprises an induciblerecombinase. In some embodiments, the inducible recombinase is aninducible Cre recombinase. In some embodiments, an inducible Crerecombinase is a Cre-ER^(T2) recombinase.

In some embodiments, the inducible recombinase is expressed in a cell ortissue, e.g., selected cells or tissues of the progeny rodent. In someembodiments, the expressed recombinase acts in a cell or tissue toinvert the altered Acvr1 exon 7 into sense orientation and to delete thefunctioning Acvr1 exon 7, thereby allowing an altered Acvr1 allelecomprising the altered exon 7 to be expressed.

Also provided herein are progeny rodents obtained from a breeding methoddisclosed herein.

In still another aspect, a genetically modified rodent animal thatexpresses an altered Acvr1 allele is used as a model for an ectopicossification disorder. In one embodiment, the ectopic ossificationdisorder is fibrodysplasia ossificans progressiva (FOP). In someembodiments, a genetically modified rodent animal that expresses analtered Acvr1 allele is used to evaluate a candidate therapeuticcompound for determining whether the candidate compound can inhibit thedevelopment of ectopic bone formation in the rodent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the Acvr1 wild type to R258G F1Ex schematics. The topschematic shows the Acvr1 genomic region after knockin and Neo deletion.The bottom schematic shows the genomic region after Cre-mediatedinversion and deletion, which places the mutant exon in its normallocation and orientation. The human exon (open box) and flankingintronic (open bar) sequences were used to reduce hairpin formation withthe inverted mouse exon/intron (striped box/striped bar) sequencesduring transcription. Additional nucleotide changes were made to thehuman exon to further reduce sequence identity with the mouse exon.

FIG. 2 shows that Acvr1^([R258G]F1Ex/+); Gt(ROSA26)Sor^(CreERt2/+) micedeveloped heterotopic ossification (HO) after tamoxifen treatment. FourAcvr1^([R258G]F1Ex/+); Gt(ROSA26)Sor^(CreErt2/+) mice were given 5 daily40 mg/kg tamoxifen injections. After 2 weeks, all mice showed HOformation.

FIG. 3 shows that an anti-Activin A blocking antibody inhibits HOformation in Acvr1^(R258G) conditional-on knock-in mice.

DETAILED DESCRIPTION

Fibrodysplasia ossificans progressiva (FOP) is an autosomal dominantdisorder of ectopic bone formation. Approximately 95% of FOP is causedby the R206H mutation in activin A type I receptor (Acvr1). However,several other mutations in either the GS or kinase domains, includingR258G, have been reported to cause atypical FOP with a more severephenotype. The two patients with the R258G mutation have profounddevelopmental abnormalities in addition to developing heterotopicossification after birth (Kaplan et al., Am J Med Genet A. 2015;167(10): 2265-2271).

Genetically modified rodents are provided that are capable of expressingan Acvr1 protein comprising an alteration that results in a disordercharacterized by ectopic bone formation, e.g., FOP. In some embodiments,rodents expressing the altered Acvr1 protein include rodents that arenot chimeric, e.g., rodents whose genomes carry a modified Acvr1comprising a conditional alteration that, once expressed, results inectopic bone formation in a rodent.

Genetically modified rodents are provided that comprise a F1Ex designthat provides for a conditional deletion of a functioning exon andreplacement of the functioning exon with an altered exon. A functioningexon encodes amino acids of a protein that is functional, performs itsexpected biological function. In some embodiments, a functioning exon isa naturally occurring, wild type exon. In some embodiments, afunctioning exon encodes the same amino acids as a wild type exon. F1Exallows for forming a conditional allele by placement of a nucleic acidsequence encoding an altered exon in the antisense strand (hereonreferred to as “inverted altered exon”) next to a functioning exon thatwill later on be deleted. By utilizing selected site-specificrecombinase recognition sites (SRRS′s), in the presence of their cognaterecombinase, the inverted altered exon is brought to the sense strand,and hence also in frame with the rest of the gene, whereas thefunctioning exon is deleted. This F1Ex approach relies on the placementof incompatible SRRS′s (e.g., lox2372 and loxP) surrounding thefunctioning and altered exons. Thus, one advantage of the F1Ex approachis that a (perinatal/embryonic) lethal mutation is not expressed unlessthe F1Ex allele is acted upon by the selected recombinase(s). Anotheradvantage of this F1Ex approach is permanent removal of the functioningexon upon exposure to the selected recombinase, and thus no invertedrepeat remains in the genome post-inversion, which eliminates thepossibility of regenerating a wild type allele. Yet another advantage ofthe F1Ex approach is the permanent fixing of the altered exon in thesense strand which results from the removal of one of each of the twodifferent types of SRRSs (e.g. one of the two lox2372 sites and one ofthe loxP sites). A functioning exon derived by humanization of awild-type rodent exon also minimizes inverted repeat sequence, thusfacilitating cloning steps and alleviating concerns of rearrangementsduring and after targeting, as well as RNA splicing artifacts duringmaturation of the corresponding mRNA. In some embodiments, asubstantially human wild type exon is used in a F1Ex design. The “humanwild type exon” or “human exon” refers to a naturally occurring,functioning exon from human. The term “substantially human wild typeexon” or “substantially human exon” include both a naturally occurring,functioning human exon, and modified forms thereof where one or more(e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or more)nucleotides of a naturally occurring human exon have been altered. Inspecific embodiments, one or more (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15 or more) nucleotides have been altered withoutchanging the encoded amino acids (essentially altered codon choices), toreduce sequence identity with the rodent exon, which further minimizesinverted repeat sequence and reduces the possibility of rearrangements.

If a rodent bearing a F1Ex allele is bred to a recombinase-containingrodent, the altered allele will be expressed in the progeny in utero;and if the alteration is a perinatal/embryonic lethal alteration, thegoal of making an animal that can be studied which expresses the allelemay be compromised. Therefore, the rodent bearing a F1Ex allele is notbred with an unregulated recombinase-containing rodent. Instead, therodent beating a F1Ex allele is bred with a rodent that expresses arecombinase, the activity of which is inducible (e.g., responsive to aninducer) (i.e., an inducible recombinase). Inducible recombinases can bemade by fusing a recombinase with a ligand binding domain of a proteinthat, when bound to its cognate ligand or functional derivatives of thecognate ligand, renders the protein functional (by stabilizing theprotein). Functional derivatives of a cognate ligand refer to compoundsthat are analogous in structure and perform substantially the samefunction (i.e., binding to the same receptor) as the cognate ligand.Examples of such ligand-binding domains include, but are not limited to,the ligand-binding domains of steroid receptors, glucocorticoidreceptors, retinoid receptors and thyroid receptors (Eilers et al.(1989) Nature 340:66-68; Picard et al, (1988) Cell 54:1073-1080). Insome embodiments, an inducible recombinase is a fusion protein betweenCre and an estrogen receptor (ER) modified with T2 mutations (encoded bya Cre-ER^(T2) allele). With this fusion protein, the Cre recombinase isinactive in the absence of a ligand for the ER (see, Indra, A. et al.(1999), Nucleic Acids Res. 27(22):4324-4327; Feil, R. et al. (1997)Biochem. Biophys. Res. Commun. 237:752-757; U.S. Pat. No. 7,112,715),and the Cre recombinase becomes active when provided with a ligand forthe ER, e.g., tamoxifen, or a functional derivative of tamoxifen. Arodent comprising a conditional allele constructed with Cre-responsiveSIMS's as described herein, and containing a Cre-ER^(T2) allele, wouldtherefore express the allele comprising the functioning exon, unless anduntil the rodent is exposed to a ligand for the ER to induce Creactivity. In this way, rodents are generated that contain a modifiedAcvr1 gene in their germline but that do not express the correspondingvariant Acvr1 protein unless and until the rodents are exposed to aligand of the ER (e.g., tamoxifen). Following exposure to the ligand,the Cre-ER^(T2) fusion protein is activated and the conditional alleleis converted to the corresponding altered allele.

A ligand can be administered to the rodent via various routes to inducethe activity of the recombinase, including parenteral and non-parenteralroutes of administration. Parenteral routes include, e.g., intravenous,intraarterial, intraportal, intramuscular, subcutaneous,intraperitoneal, intraspinal, intrathecal, intracerebroventricular,intracranial, intrapleural or other routes of injection. Non-parenteralroutes include, e.g., oral, nasal, transdermal, pulmonary, rectal,buccal, vaginal, ocular. In specific embodiments, a ligand isadministered to a rodent via intraperitoneal injection.

In various embodiments, the conversion to an altered allele isirreversible, with deletion of the functioning exon. In this manner, arodent line containing an otherwise lethal Acvr1 mutation can bemaintained essentially indefinitely, producing the desired geneticlesion and accompanying phenotype whenever desired.

The rodents provided herein include, for example, mice, rats, andhamsters. In some embodiments, the rodent is a mouse or a rat. Inspecific embodiments, the rodent is a mouse. Acvr1 is highly conservedacross species, with 8258 being conserved and at the same position formice and rats.

In some embodiments, the rodent is a mouse of a C57BL strain, forexample, a C57BL strain selected from C57BL/A, C57BL/An, C57BL/GrFa,C57BL/KaLwN, C57BL/6, C57BL/6J, C57BL/6ByJ, C57BL/6NJ, C57BL/10,C57BL/10ScSn, C57BL/10Cr, and C57BL/Ola. In other embodiments, therodent is a mouse of a 129 strain, for example, a 129 strain selectedfrom the group consisting of 129P1, 129P2, 129P3, 129X1, 129S1 (e.g.;129S1/SV, 129S1/SvIm), 129S2, 129S4, 129S5, 129S9/SvEvH, 129/SvJae,129S6 (129/SvEvTac), 1129S7, 129S8, 129T1, 129T2 (see, e.g., Festing etal. (1999), Mammalian Genome 10:836; Auerbach et al. (2000),Biotechniques 29(5):1024-1028, 10′30, 1032). In some embodiments, therodent is a mouse that is a mix of an aforementioned 129 strain and anaforementioned C57BL/6 strain. In certain embodiments, the mouse is amix (i.e., hybrid) of aforementioned 129 strains, or a mix ofaforementioned C57BL strains, or a mix of a C57BL strain and a 129strain. In certain embodiments, the mouse is a mix of a C57BL/6 strainwith a 129 strain. In specific embodiments, the mouse is a VGF1 strain,also known as F1H4, which is a hybrid of C57BL/6 and 129. In otherembodiments, the mouse is a BALB strain, e.g., BALB/c strain. In someembodiments, the mouse is a mix of a BALB strain and anotheraforementioned strain.

In some embodiments, the rodent is a rat. In certain embodiments, therat is selected from a Wistar rat, an LEA strain, a Sprague Dawleystrain, a Fischer strain, F344, F6, and Dark Agouti. In otherembodiments, the rat is a mix of two or more strains selected from thegroup consisting of Wistar, LEA, Sprague Dawley, Fischer F344, F6, andDark Agouti.

A conditional Acvr1 allele comprising the R258G variation can beengineered by utilizing the F1Ex method. See, Schnutgen, F. et al.(2003) Nat. Biotech. 21:562-565; and U.S. Pat. No. 7,205,148. F1Exemploys a pair of mutant Lox sites—referred to as a F1Ex array—that arerecognized by the same recombinase (Cre) but that do not react with oneanother, and are laid out in an A−B/[A−B] configuration, where the“[A−B]” is in the opposite strand with respect to “A−B”, to enableinversion of the DNA sequence flanked by the arrays. In someembodiments, the pair of LoxP-Lox2372 is used as the combination ofmutant Lox sites for the conditional allele described herein. These twomutant Lox sites do not exhibit cross-reactivity. The sequence that iscontained within each array—i.e., between the LoxP and Lox2372 sites ofeach array—will be deleted upon action by Cre. One embodiment of aconditional allele is illustrated in FIG. 1.

Mouse Acvr1 displays a variety of splice variants (e.g., Acvr1-201, 202,203, 204). The exons which are mutated in FOP, are shared by allprotein-coding splice variants of Acvr1. In one embodiment, disclosedherein is a conditional allele comprising a modification of exon 7(ENSMUSE00001232449) of the isoform, Acvr1-201.

An Acvr1^([R258G]F1Ex) allele can be engineered by placing the alteredversion of the amino acid 258-encoding exon of rodent Acvr1 (e.g.,ENSMUSE00001232449) in the antisense strand, so that the altered exon isnot incorporated into Acvr1's transcript. As the sequence encoded byexon 7 is required for Acvr1 function, this necessitated that afunctioning exon 7 is also incorporated into the design (exon 7 isshared by all protein-coding splice variants of Acvr1). Furthermore,since exons are not recognized as such without accessory intronicsequences, both upstream and downstream sequences of the exon are alsoincorporated into both an altered and functioning amino acid258-encoding exons. However, doing so would generate a large invertedrepeat, and such DNA structures are inherently prone to recombinationboth during the genetic engineering steps required to build thetargeting vector as well as post-targeting, in vivo (Holkers, M. et al.(2012) Nucleic Acids Res. 40:1984-1999). Furthermore, if the wild typerodent sequence of the amino acid 258-encoding exon and the upstream anddownstream intronic sequences associated with the exon were retainedintact and precede the altered rodent exon, then this wild type regioncould act as a homology arm and be utilized during targeting in therodent ES cells, thereby resulting in exclusion of the altered exon fromthe targeted allele.

Therefore, an Acvr1^([R258G]F1Ex) allele can be designed in a mannersuch that:

-   -   a) A large inverted repeat is avoided. To accomplish this, the        R258-encoding exon (e.g., ENSMUSE00001232449) as well associated        upstream and downstream intronic sequences can be replaced with        the corresponding region from human ACVR1.    -   b) The wild type rodent sequence of the R258-encoding exon        (e.g., ENSMUSE00001232449) is preserved at the protein level.        The mouse and human protein sequences encoded by exons        ENSMUSE00001232449 and ENSE00001009617, respectively, are        identical. However, where possible, the codons within the human        exon sequence (e.g., ENSE00001009617) can be altered to further        reduce the nucleotide sequence identity between the rodent and        human exons, without altering the amino acids encoded by the        exon.    -   c) The introduced human sequence is removed in its entirety upon        action by Cre. Therefore, in the “conditional-on” state—where        the Acvr1^([R258G]) mutant gene is transcribed—no human        sequences remain and hence any resulting phenotype cannot be        attributed to the presence of extraneous sequence.

More specifically, as an example, the region bounded by nucleotides58468399 to 58468770 in mmuAcvr1 (i.e., nucleotides 58468399 to 58468770of mouse Chromosome 2, GRCm38/mm 10) was replaced with a nucleic acidcomposed of nucleotides 157770252 to 157770625 of hsaACVR1 (i.e.,nucleotides 157770252 to 157770625 of human Chromosome 2, GRCh38/hg38),in a manner such that the introduced sequence, which includes hsaACVR1exon ENSE00001009617, was transcribed as part of the resulting modifiedAcvr1^([R258G]F1Ex) locus. In addition, by altering codon choice, thenucleotide sequence of human exon ENSE00001009617 was altered to reducesequence identity between the mouse and human exon, without alteringprotein coding. This introduced human sequence is referred to hereafteras hsa_e7+. Therefore, prior to inversion of the F1Ex element (alteredexon ENSMUSE00001232449 and associated upstream and downstream intronicsequences-see below), the resulting locus, Acvr1^([R258G]F1Ex), shouldfunction as wild type.

The R258G variation was modeled by altering exon ENSMUSE00001232449 inthe corresponding position, through altering the codon defined bynucleotides 58468530 to 58468532 from AGG (coding for arginine) to GGG(coding for glycine). The resulting mutant exon, along with flankingintronic sequences, were placed 3′ to hsa_e7+ and in the antisensestrand of mmuAcvr1. In addition, nucleotides 58468771-58468815 ofmmuAcvr1 were deleted in order to create a small deletion that wouldaccommodate LOA probes (Gomez-Rodriguez, J. et al. (2008) Nucleic AcidsRes. 36:e117; Valenzuela, D. et al. (2003) High-throughput engineeringof the mouse genome coupled with high-resolution expression analysis,Nat. Biotech. 21:652-659). This introduced mutated mouse sequence ishereafter referred to as mmu_e7R258G+.

Exemplary sequences of human exon 7, altered mouse exon 7 encodingR258G, and altered human exon 7, along with upstream and downstreamintronic sequences are summarized below and set forth in the SequenceListing.

SEQ ID NO Description 1 Human exon 7 2 Human exon 7 with codonalterations 3 Human sequence including codon altered exon 7 andsurrounding introns 4 Mouse altered exon 7 encoding R258G 5 Mousealtered exon 7 (in reverse as in construct) 6 Mouse sequence includingaltered exon 7 and surrounding introns 7 Mouse sequence includingaltered exon 7 and surrounding introns (in reverse as in construct) 8Rat exon 7

In order to enable Cre-dependent inversion of the mmu_e7R258G+ andsimultaneous deletion of hsa_e7+, a combination of F1Ex like Lox arrayswere used such that:

-   -   a) hsa_e7+ is preceded by a LoxP site, and followed by a Lox2372        site. In this respect, hsa_e7+ is contained with the 5′        LoxP-Lox2372 F1Ex-like array.    -   b) mmu_e7R258G+ is followed by the 3′ LoxP-Lox2372 F1Ex-like        array, but this array is engineered such that it is in a mirror        image configuration to the 5′ LoxP-Lox2372 F1Ex-like array. This        enables permanent inversion of mmu_e7R258G+ into the sense        strand by Cre.

When the resulting allele, Acvr1^([R258G]F1Ex), is exposed to Cre, thehsa_e7+ will be deleted and the mmu_e7R258G+ will be inverted into thesense strand. As a result, Acvr1^([R258G]) will be expressed in place ofAcvr1.

A targeting nucleic acid construct comprising an Acvr1 F1Ex alleledescribed above can be made for introducing the Acvr1 F1Ex allele into arodent genome. In addition to an Acvr1 F1Ex sequence (a substantiallyhuman exon 7 in sense orientation and surrounding intronic sequences, analtered rodent exon 7 encoding R258G in antisense orientation andsurrounding intronic sequences, and recombination recognition sites),the nucleic acid construct can include flanking sequences that are ofsuitable lengths and homologous to rodent Acvr1 gene sequences at anendogenous rodent Acvr1 locus so as to be capable of mediatinghomologous recombination and integration of the Acvr1 F1Ex sequence intothe endogenous rodent Acvr1 locus.

In some embodiments, a targeting nucleic acid construct comprising aAcvr1 F1Ex allele is introduced into a rodent embryonic stem (ES) cellto modify the genome of the ES cell. Both mouse ES cells and rat EScells have been described in the art. See, e.g., U.S. Pat. Nos.7,576,259, 7,659,442, and 7,294,754, and US Publ. No. 2008/0078000 A1(all of which are incorporated herein by reference) describe mouse EScells and the VELOCIMOUSE® method for making a genetically modifiedmouse; and US Publ. No. 2014/0235933 A1 and US Publ. No. 2014/0310828 A1(all of which are incorporated herein by reference) describe rat EScells and methods for making a genetically modified rat.

ES cells having an Acvr1 F1Ex allele integrated in the endogenous rodentAcvr1 locus can be selected. ES cells having an Acvr1 F1Ex alleleintegrated in the genome are then used as donor ES cells for injectioninto a pre-morula stage embryo (e.g., 8-cell stage embryo) by using theVELOCIMOUSE® method (see, e.g., U.S. Pat. Nos. 7,576,259, 7,659,442, and7,294,754, and US Publ. No. 2008/0078000 A1), or methods described in USPubl. Nos. 2014/0235933 A1 and 2014/0310828 A1. The embryo comprisingthe donor ES cells is incubated until blastocyst stage and thenimplanted into a surrogate mother to produce an F0 rodent fully derivedfrom the donor ES cells. Rodent pups bearing the F1Ex allele can beidentified by genotyping of DNA isolated from tail snips using, forexample, a loss of allele assay (Valenzuela el al., supra).

In various embodiments, a genetically modified rodent comprising anAcvr1 F1Ex allele is made by modifying a rodent ES cell to contain theF1Ex allele, and modifying the same ES cell to contain a gene encodingan inducible recombinase (e.g., Cre-ER^(T2)), and using the ES cell as adonor cell to make a rodent that contains the F1Ex allele and the geneencoding the inducible recombinase. In some embodiments, a geneticallymodified rodent comprising an Acvr1 F1Ex allele is made by using arodent ES cell that already comprises a gene encoding an induciblerecombinase (e.g., Cre-ER^(T2)), and modifying such rodent ES cell tocomprise an F1Ex allele. In other embodiments, a genetically modifiedrodent comprising an Acvr1 F1Ex allele is made and crossed with a rodentcontaining a gene encoding an inducible recombinase (e.g., Cre-ER^(T2))to obtain an offspring that contains the F1Ex allele and the geneencoding the inducible recombinase.

Acvr1^([R258G]F1Ex/+); Gt(ROSA26)Sor^(CreERt2/+) adult mice werephenotypically normal, however, body-wide activation of theR258G-encoding Acvr1 allele resulted in progressive ossification,evident radiographically as early as 2 weeks after dosing withtamoxifen, in a manner similar to that seen with the Acvr1 R206H Flexmouse described in U.S. Pat. No. 9,510,569. See also FIG. 2.

The rodents provided herein permit a better understanding of themolecular mechanisms underlying the development of ectopic bonedisorders such as FOP. In addition, such rodents may be used in thescreening and development of therapeutic compounds for the inhibition,prevention, and/or treatment of ectopic bone disorders, including FOP.

In some embodiments, a candidate therapeutic compound is tested in vivo,by administering the compound to a rodent disclosed herein, i.e., arodent carrying an Acvr1 F1Ex allele.

Candidate therapeutic compounds can be, without limitation, smallmolecule chemical compounds, antibodies, inhibitory nucleic acids, orany combination thereof. In a specific embodiment, the compound is anantibody or antigen-binding fragment thereof, e.g., an anti-Acvr1antibody or antigen-binding fragment thereof. In some embodiments, thecompound comprises an antagonist of one or more of activin receptor 1,activin receptor type 2A, and activin receptor type 2B. Any suchantagonist may comprise an antibody. In some embodiments, the compoundcomprises an antibody against activin A. An antagonist or antibodyagainst activin receptor 1, against activin receptor type 2A, againstactivin receptor type 2B, or against activin A may be any antagonist orantibody described or exemplified in U.S. Publ. No. 2018/0111983, whichis incorporated by reference herein.

Administration of the compound can be performed before, during, or afterinduction of the recombinase activity in the rodent to allow the mutantAcvr1 allele to be expressed. Candidate therapeutic compounds may bedosed via any desired route of administration including parenteral andnon-parenteral routes of administration. Parenteral routes include,e.g., intravenous, intraarterial, intraportal, intramuscular,subcutaneous, intraperitoneal, intraspinal, intrathecal,intracerebroventricular, intracranial, intrapleural or other routes ofinjection. Non-parenteral routes include, e.g., oral, nasal,transdermal, pulmonary, rectal, buccal, vaginal, ocular. Administrationmay also be by continuous infusion, local administration, sustainedrelease from implants (gels, membranes or the like), and/or intravenousinjection.

Various assays may be performed to determine the pharmacokineticproperties of administered compounds using samples obtained from rodentanimals described. Pharmacokinetic properties include, but are notlimited to, how a non-human animal processes the compound into variousmetabolites (or detection of the presence or absence of one or moremetabolites, including, but not limited to, toxic metabolites),half-life, circulating levels (e.g., serum concentration), anti-compoundresponse (e.g., antibodies), absorption and distribution, route ofadministration, routes of excretion and/or clearance of the compound.

In some embodiments, performing an assay includes determining thedifferences between a mutant rodent animal administered a compound and amutant rodent animal not administered the compound, and determiningwhether the compound can inhibit the development and/or progression ofectopic bone formation in the rodent.

The present description is further illustrated by the followingexamples, which should not be construed as limiting in any way. Thecontents of all cited references (including literature references,issued patents, and published patent applications as cited throughoutthis application) are hereby expressly incorporated by reference.

Example 1

The region bounded by nucleotides 58468399 to 58468770 in mmuAcvr1(i.e., nucleotides 58468399 to 58468770 of mouse Chromosome 2, GRCm38/mm10) was replaced with a nucleic acid composed of nucleotides 157770252to 157770625 of hsaACVR1 (i.e., nucleotides 157770252 to 157770625 ofhuman Chromosome 2, GRCh38/hg38), in a manner such that the introducedsequence, which includes hsaACVR1 exon ENSE00001009617, was transcribedas part of the resulting modified Acvr1^([R258G]F1Ex) locus. Inaddition, by altering codon choice, the nucleotide sequence of humanexon ENSE00001009617 was altered to reduce sequence identity between themouse and human exon, without altering protein coding. This introducedhuman sequence is referred to hereafter as hsa_e7+. Therefore, prior toinversion of the F1Ex element (altered exon ENSMUSE00001232449 andassociated upstream and downstream intronic sequences—see below), theresulting locus, Acvr1^([R258G]F1Ex), should function as wild type.

The R258G variation was modeled by altering exon ENSMUSE00001232449 inthe corresponding position, through altering the codon defined bynucleotides 58468530 to 58468532 from AGG (coding for arginine) to GGG(coding for glycine). The resulting altered exon, along with flankingintronic sequences, were placed 3′ to hsa_e7+ and in the antisensestrand of mmuAcvr1. In addition, nucleotides 58468771-58468815 ofmmuAcvr1 were deleted in order to create a small deletion that wouldaccommodate LOA probes (Gomez-Rodriguez, J. et al. (2008) Nucleic AcidsRes. 36:e117; Valenzuela, D. et al. (2003) High-throughput engineeringof the mouse genome coupled with high-resolution expression analysis,Nat. Biotech. 21:652-659). This introduced mutated mouse sequence ishereafter referred to as mmu_e7R258G+.

In order to enable Cre-dependent inversion of the mmu_e7R258G+ andsimultaneous deletion of hsa_e7+, a combination of F1Ex like Lox arrayswere used such that:

-   -   a) hsa_e7+ was preceded by a LoxP site, and followed by a        Lox2372 site. In this respect, hsa_e7+ was contained with the 5′        LoxP-Lox2372 F1Ex-like array.    -   b) mmu_e7R258G+ was followed by the 3′ LoxP-Lox2372 F1Ex-like        array, but this array was engineered such that it was in a        mirror image configuration to the 5′ LoxP-Lox2372 F1Ex-like        array. This enabled permanent inversion of mmu_e7R258G+ into the        sense strand by Cre.

When the resulting allele, Acvr1^([R258G]F1Ex), is exposed to Cre, thehsa_e7+ will be deleted and the mmu_e7R258G+ will be inverted into thesense strand. As a result, Acvr1^([R258G]) will be expressed in place ofAcvr1.

Homozygous Acvr1^([R258G]F1Ex) mice were born at an expected Mendalianratio suggesting that splicing was normal in this mouse and wild typeAcvr1 was being expressed.

Example 2

Inducing FOP in Acvr1^([R258G]F1Ex) mice

To enable time-controlled yet whole body inversion of theAcvr1^([R258G]F1Ex) allele, Acvr1^([R258G]F1Ex) mice were mated withGt(ROSA26)Sor^(CreERT2/+) mice to generate Acvr1^([R258G]F1Ex);Gt(ROSA26)Sor^(CreERT2/+). These were maintained in heterozygosity on amixed—C57BL/6NTac-129S6/SvEvTac background. All experiments wereperformed in accordance with the Institutional Animal Care and UseCommittee of Regeneron. Both male and female mice were used between 8and 11 weeks of age, however mice were aged and sex matched betweengroups. No age or sex related phenotypes were noted. The model wasinitiated by inversion of the R258G-encoding exon into the sense strand,which was accomplished by treating Acvr1^([R258G]F1Ex);Gt(ROSA26)Sor^(CreERT2/+) mice with 40 mg/kg of tamoxifen (Sigma) in oilintraperitoneally (i.p.) daily for 5 days (to activate CreER^(T2)). Toassess heterotopic bone formation, mice were anesthetized by isofluoraneand whole body-scanned, with a field of view at 60 mm×120 mm, using invivo μCT (Quantum FX, PerkinElmer, Hopkinton, Mass., USA). The X-raysource was set to a current of 160 μA, voltage of 90 kVp, with a voxelsize at 120 or 240 μm.

Acvr1^([R258G]F1Ex); Gt(ROSA26)Sor^(CreERT2/+) adult mice werephenotypically normal; however, body-wide activation of the R258G mutantAcvr1 allele resulted in progressive ossification, evidentradiographically as early as 2 weeks after dosing with tamoxifen, in amanner similar to that seen with the Acvr1R206HFlex mouse described inU.S. Pat. No. 9,510,569. See also FIG. 2.

Antibody Dosing of Mice

For treatment studies, Acvr1^([R258G]F1Ex); Rosa^(CreERT2) mice wereseparated to ensure age and sex matching across groups, treatments wereinitiated on the same day as tamoxifen administration. Mice wereinjected subcutaneously (s.c.) with 25 mg/kg of either a neutralizingantibody generated against human Activin A (U.S. Patent Application2015/0037339) or an isotype control antibody weekly for 6 weeks.Heterotopic bone formation was monitored weekly by in vivo μCT imaging.

As shown in FIG. 3, mice receiving an isotype control antibody developedHO at 4 weeks, and mice receiving an Activin A blocking antibody had nodetectable HO formation above μCT background after 4 weeks.

In summary, conditional-on knock-in alleles of ACVR1^(R206H) orACVR1^(R258G) faithfully model Fibrodysplasia Ossificans Progressiva(FOP) in rodents such as mice; and an Activin A neutralizing antibodyblocks the development of heterotopic ossification such rodent model.

Further Embodiments

-   1. A genetically modified rodent, whose genome comprises a modified    rodent Acvr1 gene within an endogenous rodent Acvr1 locus, wherein    the modified rodent Acvr1 gene comprises    -   a) a substantially human ACR1 exon 7 in sense orientation        flanked by a first pair of site-specific recombinase recognition        sites (SRRS′), wherein the substantially human ACWR1 exon 7        encodes the same amino acids as a human ACVR1 exon 7; and    -   b) an altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by a second pair of SRRS′ that are        different from the first pair of SRRS′;    -   wherein the first and second SRRS′ are oriented so that a        recombinase can invert the mutant rodent Acvr1 exon 7 into sense        orientation, delete the substantially human ACR1 exon 7, and        allow an altered Acvr1 allele comprising the altered rodent        Acvr1 exon 7 to be expressed.-   2. The genetically modified rodent of item 1, wherein the    substantially human ACVR1 exon 7 differs from the human ACVR1 exon 7    by at least one nucleotide and has a reduced sequence identity with    the altered rodent Acvr1 exon 7 as compared to the sequence identity    between the human ACVR1 exon 7 and the altered rodent Acvr1 exon 7.-   3. The genetically modified rodent of item 1, wherein the gene    encoding the recombinase is in the genome of the genetically    modified rodent, and the activity of the recombinase is inducible.-   4. The genetically modified rodent according to any one of items    1-3, wherein the recombinase is Cre.-   5. The genetically modified rodent of item 4, wherein the Cre is    fused to a ligand binding domain of an estrogen receptor (ER) such    that the activity of the Cre is induced by ligand binding to the ER.-   6. The genetically modified rodent of item 5, wherein the ligand    binding domain of the ER comprises T2 mutations.-   7. The genetically modified rodent of item 5 or 6, wherein the    ligand is tamoxifen.-   8. The genetically modified rodent of any one of items 1-7, wherein    the genetically modified rodent is homozygous for the modified Acvr1    gene.-   9. A mutant rodent derived from the genetically modified rodent of    any of items 1-8, wherein the mutant rodent has a genome comprising    an altered Acvr1 allele which comprises the altered exon 7 in sense    orientation, and wherein the altered Acvr1 allele is expressed in    the mutant rodent resulting in ectopic bone formation.-   10. The rodent according to any of the preceding items, selected    from a mouse or a rat.-   11. A nucleic acid, comprising a modified rodent Acvr1 gene, wherein    the modified rodent Acvr1 gene comprises    -   a) a substantially human ACVR1 exon 7 in sense orientation        flanked by a first pair of site-specific recombinase recognition        sites (SRRS′), wherein the substantially human ACVR1 exon 7        encodes the same amino acids as a human ACVR1 exon 7; and    -   b) an altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by a second pair of SRRS′ that are        different from the first pair of SRRS′;    -   wherein the first and second SRRS′ are oriented so that a        recombinase can invert the mutant rodent Acvr1 exon 7 into sense        orientation, delete the substantially human ACVR1 exon 7.-   12. The nucleic acid of item 11, wherein the substantially human    ACVR1 exon 7 differs from the human ACVR1 exon 7 by at least one    nucleotide and has a reduced sequence identity with the altered    rodent Acvr1 exon 7 as compared to the sequence identity between the    human ACVR1 exon 7 and the altered rodent Acvr1 exon 7.-   13. The nucleic acid of item 11, wherein the activity of the    recombinase is inducible.-   14. The nucleic acid of any one of items 11-13, wherein the    recombinase is Cre.-   15. The nucleic acid of item 14, wherein the Cre is fused to a    ligand binding domain of an estrogen receptor (ER) such that the    activity of Cre is induced by ligand binding to the ER.-   16. The nucleic acid of item 15, wherein the ligand binding domain    of the ER comprises T2 mutations.-   17. The nucleic acid of item 15 or 16, wherein the ligand is    tamoxifen.-   18. The nucleic acid of any one of items 11-17, wherein the rodent    is a mouse or a rat.-   19. A rodent genome comprising the nucleic acid of item 11 or 12.-   20. The rodent genome of item 19, further comprising a gene encoding    the recombinase that recognizes the SRRS′ and inverts the altered    exon.-   21. The rodent genome of item 20, wherein the activity of the    recombinase is inducible.-   22. The rodent genome of item 21, wherein the recombinase is Cre.-   23. The rodent genome of item 22, wherein the Cre is fused to a    ligand binding domain of an estrogen receptor (ER) such that the    activity of the Cre is induced by ligand binding to the ER.-   24. The rodent genome of item 23, wherein the ligand binding domain    of the ER comprises T2 mutations.-   25. The rodent genome of item 23 or 24, wherein the ligand is    tamoxifen.-   26. The rodent genome of any one of items 19-25, wherein the rodent    genome is homozygous for for the modified Acvr1 gene.-   27. The rodent genome of any one of items 19-26, wherein the rodent    is a mouse or a rat.-   28. An isolated rodent tissue or cell comprising the rodent genome    of any one of items 19-27.-   29. The isolated rodent tissue or cell of item 28, wherein the    rodent is a mouse or a rat.-   30. The isolated rodent tissue or cell of item 28 or 29, wherein the    rodent cell is an embryonic stem cell.-   31. A nucleic acid construct for targeted modification of an Acvr1    gene in a rodent genome, comprising:    -   a) a substantially human ACVR1 exon 7 in sense orientation        flanked by a first pair of site-specific recombinase recognition        sites (SRRS′), wherein the substantially human ACVR1 exon 7        encodes the same amino acids as a human ACVR1 exon 7; and    -   b) an altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by a second pair of SRRS′ that are        different from the first pair of SRRS′;    -   wherein the first and second SRRS′ are oriented so that a        recombinase can invert the mutant rodent Acvr1 exon 7 into sense        orientation, delete the substantially human ACVR1 exon 7, and        allow an altered Acvr1 allele comprising the altered rodent        Acvr1 exon 7 to be expressed.-   32. The nucleic acid construct of item 31, wherein the substantially    human ACVR1 exon 7 differs from the human ACVR1 exon 7 by at least    one nucleotide and has a reduced sequence identity with the altered    rodent Acvr1 exon 7 as compared to the sequence identity between the    human ACVR1 exon 7 and the altered rodent Acvr1 exon 7.-   33. The nucleic acid construct of item 31, wherein the first and    second pairs of SRRS are Lox2372 and LoxP or vice versa.-   34. The nucleic acid construct of item 31, wherein the recombinase    is Cre.-   35. The nucleic acid construct of item 34, wherein the Cre is fused    to an estrogen receptor (ER) such that the activity of the Cre is    induced by ligand binding to the ER.-   36. The nucleic acid construct of item 35, wherein the ER comprises    T2 mutations.-   37. The nucleic acid construct of item 35, wherein the ligand is    tamoxifen.-   38. The nucleic acid construct according to any one of item 31-37,    wherein the rodent is selected from a mouse or a rat.-   39. A method of making a genetically modified rodent, comprising    modifying a rodent genome to comprise a modified rodent Acvr1 gene    within an endogenous rodent Acvr1 locus, wherein the modified rodent    Acvr1 gene comprises    -   a) a substantially human ACVR1 exon 7 in sense orientation        flanked by a first pair of site-specific recombinase recognition        sites (SRRS′), wherein the substantially human ACVR1 exon 7        encodes the same amino acids as a human ACVR1 exon 7; and    -   b) an altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by a second pair of SRRS′ that are        different from the first pair of SRRS′;    -   wherein the first and second SRRS′ are oriented so that a        recombinase can invert the altered rodent Acvr1 exon 7 into        sense orientation, delete the substantially human ACVR1 exon 7,        and allow an altered Acvr1 allele comprising the altered rodent        Acvr1 exon 7 to be expressed.-   40. The method of item 39, wherein the rodent genome is modified by    a process comprising:    -   a) introducing a nucleic acid construct into a rodent embryonic        stem (ES) cell, wherein the nucleic acid construct comprises:    -   the substantially human ACVR1 exon 7 in sense orientation        flanked by the first pair of SRRS′, and    -   the altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by the second pair of SRRS′;    -   wherein the nucleic acid construct targets the endogenous rodent        Acvr1 locus resulting in the modified rodent Acvr1 gene within        the endogenous rodent Acvr1 locus;    -   b) obtaining a genetically modified rodent ES cell whose genome        comprises the modified rodent Acvr1 gene; and    -   c) making a genetically modified rodent by using the genetically        modified rodent ES cell from b).-   41. The method of item 39 or 40, wherein the genetically modified    rodent is homozygous for the modified Acvr1 gene.-   42. The method of item 40 or 41, wherein the rodent ES cell further    comprises a gene encoding the recombinase.-   43. The method of any one of items 39-42, wherein the activity of    the recombinase is inducible.-   44. The method according to any one of items 39-43, wherein the    recombinase is Cre.-   45. The method of item 44, wherein the Cre is fused to a ligand    binding domain of an estrogen receptor (ER) such that the activity    of the Cre is induced by ligand binding to the ER.-   46. The method of item 45, wherein the ligand binding domain of the    ER comprises T2 mutations.-   47. The method of item 45 or 46, wherein the ligand is tamoxifen.-   48. The method of any one of items 43-47, further comprising    inducing the activity of the recombinase in a cell or tissue of the    rodent, wherein the recombinase inverts the altered exon 7, deleting    the substantially human exon 7, thereby allowing an altered Acvr1    allele comprising the altered exon 7 to be expressed in the cell or    tissue.-   49. The method according to any one of items 40-48, wherein the    rodent is selected from a mouse or a rat.-   50. A method of breeding, comprising breeding a first rodent whose    genome comprises a modified rodent Acvr1 gene with a second mouse,    resulting in a progeny rodent whose genome comprises the modified    rodent Acvr1 gene, wherein the modified rodent Acvr1 gene comprises:    -   a) a substantially human ACVR1 exon 7 in sense orientation        flanked by a first pair of site-specific recombinase recognition        sites (SRRS′), wherein the substantially human ACVR1 exon 7        encodes the same amino acids as a human ACVR1 exon 7; and    -   b) an altered rodent Acvr1 exon 7 encoding R258G in antisense        orientation, flanked by a second pair of SRRS′ that are        different from the first pair of SRRS′;    -   wherein the first and second SRRS′ are oriented so that a        recombinase can invert the altered rodent Acvr1 exon 7 into        sense orientation, delete the substantially human ACVR1 exon 7,        and allow an altered Acvr1 allele comprising the altered rodent        Acvr1 exon 7 to be expressed.-   51. The method of item 50, wherein the first rodent is homozygous    for the modified rodent Acvr1 gene.-   52. The method of item 50 or 51, wherein the second rodent comprises    an inducible recombinase.-   53. The method of item 52, wherein the inducible recombinanse is an    inducible Cre recombinase.-   54. The method of item 53, wherein the inducible Cre recombinase    comprises a tamoxifen-inducible Cre-ER recombinase.-   55. The method of any one of items 52-54, further comprising    inducing the inducible recombinase in a cell or tissue in the    progeny rodent, such that the induced recombinase inverts the    altered rodent Acvr1 exon 7 into sense orientation and delete the    substantially human ACVR1 exon 7 in the cell or tissue, thereby    producing an altered Acvr1 allele comprising the altered rodent    Acvr1 exon 7.-   56. The method of any one of items 50-55, wherein the rodent is a    mouse or a rat.-   57. A progeny rodent produced according to the method of any one of    items 50-56.-   58. A method of testing a candidate therapeutic compound for    treating ectopic bone formation, comprising:    -   providing a genetically modified rodent according to any one of        items 1-8 and 10;    -   inducing the activity of the recombinase in the rodent to allow        an altered Acvr1 allele comprising the altered rodent Acvr1 exon        7 to be expressed;    -   administering the candidate compound to the rodent; and    -   determining whether the candidate compound inhibits the        development of ectopic bone formation in the rodent.-   59. The method of item 58, wherein the candidate compound is    administered to the rodent before, during, or after the induction of    the recombinase activity.-   60. The method of item 58, wherein the candidate compound is a small    molecule chemical compound.-   61. The method of item 58, wherein the candidate compound is a    nucleic acid.-   62. The method of item 58, wherein the candidate compound is an    antibody or antigen-binding fragment thereof.-   63. The method of item 62, wherein the antibody or antigen-binding    fragment thereof is an antibody or antigen-binding fragment thereof    against activin receptor 1.-   64. The method of item 62, wherein the antibody or antigen-binding    fragment thereof is an antibody or antigen-binding fragment thereof    against activin receptor type 2A.-   65. The method of item 62, wherein the antibody or antigen-binding    fragment thereof is an antibody or antigen-binding fragment thereof    against activin receptor type 2B.-   66. The method of item 62, wherein the antibody or antigen-binding    fragment thereof is an antibody or antigen-binding fragment thereof    against activin A.-   67. The method according to any one of items 58-66, wherein the    rodent is a mouse or a rat.

What is claimed is:
 1. A genetically modified rodent whose genomecomprises a modified Activin A receptor type 1 (ACVR1) gene within anendogenous rodent ACVR1 locus, wherein the modified ACVR1 genecomprises: a) an ACVR1 exon 7 in sense orientation flanked by a firstpair of site-specific recombinase recognition sites (SRRSs), wherein theACVR1 exon 7 encodes the same amino acid sequence as a wild-type exon 7of a human ACVR1 gene; and b) a mutant exon 7 of a rodent ACVR1 gene inantisense orientation encoding a R258G substitution flanked by a secondpair of SRRSs that are different from the first pair of SRRSs; whereinthe exon 7 in sense orientation: i) is the wild-type exon 7 of the humanACVR1 gene; or ii) has decreased sequence identity to the mutant exon 7of the rodent ACVR1 gene as compared to the wild-type exon 7 of thehuman ACVR1 gene; and wherein the first and second pairs of SRRSs areoriented so that a recombinase can invert the mutant exon 7 into senseorientation, delete the exon 7 in sense orientation, and allow a mutantACVR1 gene comprising the mutant exon 7 to be expressed resulting inectopic bone formation.
 2. The rodent of claim 1, wherein the exon 7 insense orientation has decreased sequence identity to the mutant exon 7of the rodent ACVR1 gene as compared to the wild-type exon 7 of thehuman ACVR1 gene.
 3. The rodent of claim 1, wherein the genome of therodent further comprises a nucleic acid sequence encoding therecombinase, and wherein the activity of the recombinase is inducible.4. The rodent of claim 3, wherein the recombinase is Cre.
 5. The rodentof claim 4, wherein the Cre is fused to a ligand binding domain of anestrogen receptor (ER) such that the activity of the Cre is induced byligand binding to the ER.
 6. The rodent of claim 5, wherein the ligandbinding domain of the ER comprises T2 mutations.
 7. The rodent of claim5, wherein the ligand is tamoxifen.
 8. The rodent of claim 1, whereinthe genetically modified rodent is homozygous for the modified ACVR1gene.
 9. An offspring rodent derived from the rodent of claim 1, whereinthe offspring rodent i) has a genome comprising the mutant ACVR1 genecomprising the mutant exon 7, and ii) expresses the mutant ACVR1 generesulting in ectopic bone formation.
 10. The rodent of claim 1, whereinthe rodent is a mouse or rat.
 11. An isolated genetically modifiedrodent cell or tissue whose genome comprises a modified Activin Areceptor type 1 (ACVR1) gene within an endogenous rodent ACVR1 locus,wherein the modified ACVR1 gene comprises: a) an ACVR1 exon 7 in senseorientation flanked by a first pair of site-specific recombinaserecognition sites (SRRSs), wherein the ACVR1 exon 7 encodes the sameamino acid sequence as a wild-type exon 7 of a human ACVR1 gene; and b)a mutant exon 7 of a rodent ACVR1 gene in antisense orientation encodinga R258G substitution flanked by a second pair of SRRSs that aredifferent from the first pair of SRRSs; wherein the exon 7 in senseorientation: i) is the wild-type exon 7 of the human ACVR1 gene; or ii)has decreased sequence identity to the mutant exon 7 of the rodent ACVR1gene as compared to the wild-type exon 7 of the human ACVR1 gene; andwherein the first and second pairs of SRRSs are oriented so that arecombinase can invert the mutant exon 7 into sense orientation, deletethe exon 7 in sense orientation, and allow a mutant ACVR1 genecomprising the mutant exon 7 to be expressed resulting in ectopic boneformation.
 12. The isolated genetically modified rodent cell or tissueof claim 11, wherein the rodent is a mouse or a rat.
 13. The isolatedgenetically modified rodent cell or tissue of claim 11, wherein the cellis an embryonic stem (ES) cell.
 14. A method of making a geneticallymodified rodent whose genome comprises a modified Activin A receptortype 1 (ACVR1) gene, the method comprising: A) introducing a nucleicacid construct into an isolated rodent embryonic stem (ES) cell; B)injecting the ES cell into a recipient rodent embryo; C) implanting theembryo into a surrogate female such that a genetically modified rodentwhose genome comprises a modified ACVR1 gene within an endogenous rodentACVR1 locus is obtained, wherein the modified ACVR1 gene comprises: i)an ACVR1 exon 7 in sense orientation flanked by a first pair ofsite-specific recombinase recognition sites (SRRSs), wherein the ACVR1exon 7 encodes the same amino acid sequence as a wild-type exon 7 of ahuman ACVR1 gene; and ii) a mutant exon 7 of a rodent ACVR1 gene inantisense orientation encoding a R258G substitution flanked by a secondpair of SRRSs from are different from the first pair of SRRSs; whereinthe exon 7 in sense orientation: a) is the wild-type exon 7 of the humanACVR1 gene; or b) has decreased sequence identity to the mutant exon 7of the rodent ACVR1 gene as compared to the wild-type exon 7 of thehuman ACVR1 gene; and wherein the first and second pairs of SRRSs areoriented so that a recombinase can invert the mutant exon 7 into senseorientation, delete the exon 7 in sense orientation, and allow a mutantACVR1 gene comprising the mutant exon 7 to be expressed resulting inectopic bone formation.
 15. The method of claim 14, wherein the rodentwhose genome comprises the modified ACVR1 gene obtained is homozygousfor the modified ACVR1 gene.
 16. The method of claim 14, wherein thegenome of the rodent ES cell comprises a nucleic acid sequence encodingthe recombinase.
 17. The method of claim 14, wherein the activity of therecombinase is inducible.
 18. The method of claim 17, further comprisinginducing the activity of the recombinase in a cell or tissue of therodent whose genome comprises the modified ACVR1 gene such that themutant exon 7 is inverted into sense orientation, the exon 7 in senseorientation is deleted, and the mutant ACVR1 gene comprising the mutantexon 7 is expressed and causes ectopic bone formation in the rodent. 19.The method of claim 14, wherein the recombinase is Cre.
 20. The methodof claim 19, wherein the Cre is fused to a ligand binding domain of anestrogen receptor (ER) such that the activity of the Cre is induced byligand binding to the ER.
 21. The method of claim 20, wherein the ligandbinding domain of the ER comprises T2 mutations.
 22. The method of claim20, wherein the ligand is tamoxifen.
 23. The method of claim 14, whereinthe rodent is a mouse or rat.